Sub-models in scenes?
Yes, this is possible through linking objects.

In the master scene file simply go to file > link or hit Ctrl + Alt + O, choose the file you want to link an object from, go to object and select the object.  Then when you change the files containing the objects they will change in the master file.

Linked objects will be highlighted with light blue.


Something to keep in mind is that you can't edit the linked objects in the master file which makes it cumbersome to make small adjustments.